(1) Field of the Invention
The present invention relates to a pneumatic tire having a radial structure. More particularly, the invention relates to a pneumatic tire in which the structure of the shoulder region of the tire is improved.
(2) Description of the Prior Art
In a radial tire, the side wall portion has a higher flexibility and a larger quantity of deformation than in a tire of a bias structure, but the tire tread portion has a high rigidity and is hardly deformable. Accordingly, the tire shoulder portion located intermediately between the side wall portion and the tread portion undergoes large discontinuous stress or strain by the inner pressure and load. Moreover, a carcass ply arranged in the radial direction, which acts as a tension member in the region of this soulder portion, inevitably receives high shearing. Accordingly, shearing stress or strain is caused in the interface of the carcass ply and stress or strain is always imposed on the interface of the carcass ply by repeated bending and deformation during driving. Therefore, fatigue is readily caused in a rubber covering the carcass cords in the region of the shoulder portion, resulting in reduction of the bonding strength between the carcass cord and rubber. This reduction of the bonding strength proceeds as a function of the level of the shearing force and the driving distance. When the bonding strength is reduced below the level of the shearing force acting between the carcass cord and the covering rubber, peeling is caused between the carcass cord and the covering rubber. That is, a trouble so-called "separation" is caused. Occurrence of this trouble is especially conspicuous in case of a metal cord carcass.
As means for eliminating the above disadvantage, there have been proposed a technique of improving bonding between a cord and a rubber, a technique of plating a metal cord for improving its bonding to a cord, a technique of arranging a synthetic fiber cord layer to protect a carcass ply in the tire shoulder portion as disclosed in the specification of U.S. Pat. No. 3,789,900 and a technique of making a shape or profile of a carcass ply in agreement with a natural sectional profile so as to reduce the initial shearing stress as the time of application of the inner pressure as disclosed in the specifications of U.S. Pat. Nos. 3,757,844 and 4,037,637.
In each of these conventional pneumatic tires having a radial structure, the problem of occurrence of a separation trouble between a carcass ply, especially a metal cord carcass ply, and a covering rubber in the region of the tire shoulder portion, which is caused while the tire is used for a long time, has not been completely solved. This problem is especially serious in tires for use in high-load vehicles such as large trucks and busses.